(1) Field of the Invention
This invention relates to a new class of terpolymers for use as high strain electrostrictive polymer films. More particularly, the invention relates to a class of electrostrictive terpolymers comprising vinylidene fluoride (VDF), trifluoroethylene (TrFE) and at least one monomer having at least one bulky halogen atom side group. The monomer is preferably a chloro-monomer such as chlorofluoroethylene (CFE) or chlorotrifluoroethylene (CTFE). The chlorofluoroethylene (CFE) is preferably 1-chloro-2-fluoroethylene or 1-chloro-1-fluoroethylene.
(2) Description of the Prior Art
Many research activities in the past decade have focused on vinylidene fluoride-trifluoroethylene (VDF-TrFE) copolymers with the goal of reducing the energy barrier for ferroelectric-paraelectric phase transition and generating large and fast electric-induced mechanical responses at ambient temperatures. The close connection between the crystalline structure and electric properties led to many attempts to alter copolymer morphology by mechanical deformation, electron-radiation, crystallization, etc.
One of the main methods of processing or converting polymers into electrostrictive polymers has been by electron irradiation. Electron irradiation is the exposure to high-energy electrons. Electron irradiation of polymer films serves to break up the large crystalline regions of the polymer films into polar micro-regions resulting in a high-strain electrostrictive material.
Electrostriction is the high strains displayed by certain materials when stressed by electric fields. The magnitude of the electrostrictive strain can be described by the following equation:S=QP2,where Q is the electrostrictive coefficient and P is the polarization of the material.
Ferroelectric polymers such as poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] films, previously annealed, can be converted into electrostrictive polymers by exposure to high energy electron bombardment. Electron bombardment of high crystalline P(VDF-TrFE) films break up the long-range ferroelectric region into polar micro-domains thereby broadening the ferroelectric-to-paraelectric transition and moving the transition to a lower temperature where high strains can be observed when the films are driven by large electric fields. These strains in the polymer films caused by electron bombardment and the ensuing effects on the polymer structure can be characterized by differential scanning calorimetry, X-ray diffraction and infrared spectroscopy.
Ferroelectric polymers can contain various trans and gauche configurations, including form I(β), II(α), and III(γ). In form I, the chains exhibit an all-trans configuration. In form II, the packed chains exhibit the tgtg′ (t=trans; g,g′=gauche) conformation, resulting in a nonpolar crystallite. In form III, the chains exhibit tttgtttg′ conformation, resulting in a monoclinic lattice and a polar cell.
Electron irradiation, i.e., electron bombardment, of these ferroelectric polymers converts the polar all-trans form I(β), long-range ferroelectric regions of annealed P(VDF-TrFE) films into nanoregions consisting of coexisting I(β), II(α), and III(γ) crystallites, preferably having Curie (polar-nonpolar crystalline phase) transition at ambient temperatures. The polarization of these regions give rise to a macroscopic polarization and increase in the dielectric constant. The macroscopic polarization provides an increase in dielectric constant, large strains, much improved coupling constants and large (d33) signal piezoelectric constant.
Where the electrostrictive strains of materials are high enough, materials having this property offer great promise in applications such as sensors, underwater sonar transduction, polymeric actuators, artificial muscles, and robotics. In these and other applications, the high strain electrostrictive materials provide higher/greater sensitivity, more powerful signals and more efficient energy conversion.
However, electron irradiation, i.e., electron bombardment, is cumbersome and expensive. Electron irradiation is also a slow process because a large dose of radiation is needed to achieve electrostrictive properties. Other disadvantages associated with using electron irradiation which cause it to be a slow process is that: (1) only a limited thickness of films can be irradiated at a time; (2) the electron beam used is narrow, while film size varies and can be much wider than the electron beam; and (3) a vast-fluctuation in conditions may exist throughout the electron irradiation process.
The prior art discloses various polymers such as Nakamura et al., U.S. Pat. No. 4,543,293, which is said to disclose a piezoelectric polymer comprising vinylidene fluoride, trifluoroethylene and vinyl fluoride.
Also known in the prior art is Pantelis, U.S. Pat. No. 4,557,880, which is said to disclose a piezoelectric film made from vinylidene fluoride and tetrafluoroethylene and/or trifluoroethylene.
Also known in the prior art is Sako et al., U.S. Pat. No. 4,577,005, which is said to disclose a polymeric dielectric material comprising a terpolymer which comprises vinylidene fluoride, trifluoroethylene and hexafluoropropylene that is heat treated.
Also known in the prior art is Preis, U.S. Pat. No. 4,778,867, which is said to disclose a ferroelectric random copolymer consisting essentially of vinylidene fluoride and trifluoroethylene that is heat treated.
Also known in the prior art is Inukai et al., U.S. Pat. No. 5,087,679, which is said to disclose a polymeric dielectric which comprises vinylidene fluoride, trifluoroethylene and chlorotrifluoroethylene.
Also known in the prior art is Chung et al., U.S. Pat. No. 6,355,749, which is said to disclose a ferroelectric terpolymer comprising vinylidene fluoride, trifluoroethylene and chlorotrifluoroethylene or hexafluoropropene.
Also known in the prior art is Gervasi et al., U.S. Pub. No. 2002/0132074, which is said to disclose a fluoroelastomer terpolymer comprising vinylidene fluoride, hexafluoropropylene and tetrafluoroethylene or chlorotrifluoroethylene.
Other ferroelectric and electrostrictive polymers and methods of altering polymer morphology may be known. However, these polymers and methods, along with those above, have various shortcomings. These shortcomings are addressed by the present invention. As such, the present invention provides electrostrictive polymers using an alternative method of altering copolymer morphology, without the need for electron irradiation.